The company's founders Jan Koum, 38, and Brian Acton, 42, shun the media spotlight and are much older than your typical college dropout-turned CEO. And at a time when social media companies are focusing on advertising to generate revenue, WhatsApp rejects the idea of showing ads to the 450 million people who use its mobile messaging app.

The whopping $19 billion that Facebook is paying for the service is also unusual, even as other startups with no profit, or even revenue, are commanding sky-high valuations.

Koum and Acton are at the center of the largest buyout deal ever for a venture-backed company. How did two former Yahoo engineers who witnessed the late `90s dot-com boom - and bust - create the world's hottest app and make 10-year-old Facebook seem a tad grizzled?

''Jan keeps a note from Brian taped to his desk that reads `No Ads! No Games! No Gimmicks!' It serves as a daily reminder of their commitment to stay focused on building a pure messaging experience,'' wrote Sequoia Capital partner Jim Goetz in a blog post about Thursday's deal. Sequoia is WhatsApp's sole venture capital investor.

The Ukraine-born Koum, WhatsApp's CEO, move to the U.S. when he was 16. Acton was born in Michigan.

''We're the most atypical Silicon Valley company you'll come across,'' Acton told Wired in a December interview that the magazine will publish next month in its U.K edition. ''We were founded by thirtysomethings; we focused on business sustainability and revenue rather than getting big fast; we've been incognito almost all the time; we're mobile first; and we're global first.''

The pair started WhatsApp in 2009, two years after they left their jobs at Yahoo Inc. and five years after Facebook got its start in Mark Zuckerberg's Harvard dorm room. The service is simple. People use it to send text, photo or video messages to their contacts, bypassing text messaging charges and other fees from wireless carriers.

''WhatsApp is simple, secure, and fast. It does not ask you to spend time building up a new graph of your relationships; instead, it taps the one that's already there. Jan and Brian's decisions are fueled by a desire to let people communicate with no interference,'' writes Goetz, who along with Sequoia also stands to reap a hefty sum from the deal.

Much like Zuckerberg did during Facebook's early years, WhatsApp's founders shun ads. But unlike Facebook, which